1. Technical Field of the Invention
This disclosure relates generally to electronic circuits, and in particular, to circuits for nonlinear headroom compensation.
2. Description of the Related Art
Conventional solutions for wide-swing output stage designs use common-source output gain stages with a passive current source as a high-impedance load. While these conventional solutions may offer wide-swing operation, it is generally at the expense of compromising other important operating characteristics.
FIG. 1 is a circuit diagram illustrating a conventional solution for headroom compensation that utilizes a common source p-channel driver transistor Q driven from a low gain first or second stage. An output from the low gain first or second stage is coupled to the gate of the driver transistor Q. Transistor Q has a gate and source that are coupled through a series-connected resistor R and capacitor C. The drain of transistor Q is connected to a power supply voltage Vpwr. The sinking capability is provided by a current source Isink.
There are several disadvantages to conventional solutions represented by the circuit of FIG. 1. For example, the p-channel driver transistor Q must be made physically large in order to result in wide-swing output performance over the intended operating range. This results in reduced small-signal bandwidth performance due to the increased capacitive loading on the stage previous to the output amplifier stage, as well as degraded large-signal recovery performance due to slew rate limiting from the increased capacitive loading. Another disadvantage is the inability to drive the common source p-channel driver transistor Q from a high-performance folded cascode gain stage without an intervening level-shifting circuit. Yet another disadvantage is the inability to provide nonlinear feedback control, which sharply increases the need for headroom compensation as the output p-channel driver transistor Q enters the triode (or ohmic) region.
It would be desirable to have an architecture and method for headroom compensation that includes dynamic transconductance (gm) enhancement of an output stage p-channel driver device using nonlinear local feedback control to automatically compensate for reduced headroom conditions of operation. Embodiments of the invention fulfill these requirements in addition to addressing other disadvantages inherent to the conventional solutions discussed above.